Method of impact extruding



United States Patent 3,407,062 METHOD OF IMPACT EXTRUDING Russell E.Matthews and George S. Foerster, Midland,

Mich., assignors to The Dow Chemical Company, Midland, Mich., acorporation of Delaware No Drawing. Filed Jan. 5, 1967, Ser. No. 607,36912 Claims. (Cl. 75-214) ABSTRACT OF THE DISCLOSURE The improvement inmethod of producing impact extruded articles from a light metal alloyselected from the group consisting of magnesium-base alloys andaluminum-base alloys according to which there is provided light metalalloy in particulate form having at least one-third of the particleswith a longest to shortest dimension ratio of at least about two andsubjecting a charge of such particulate metal to an impact extrusionoperation at a temperature of at least about 600 F. The particulatemetal includes chips prepared from castings as well as flattenedatomized pellets.

The invention relates to an improvement in a method of impact extrudingand more particularly relates to a meth- 0d of making impact extrusionsfrom pelletized magnesium or aluminum-base alloys.

For the purposes of the following description and the appended claimsmagnesium-base alloys or aluminumbase alloys are each understood tocontain at least 70 percent by weight of the base metal.

'In the making of an impact extrusion, the work piece or blank is placedin a die cavity and forced to flow around a punch as the punch movestoward the bottom of the cavity. The top of a can formed in this manneris not heavily worked and heretofore has exhibited a tendency to crackif the extrusion blank is made from particulate metal. When cracksappear, they tend to propagate vertically towards the bottom of the can.The cracks can be removed by trimming off the top of the can but metalefficiency is markedly reduced by such trimming.

It is a principal object of the present invention to overcome thedifliculties heretofore encountered in making impact extrusions using adie blank fabricated from particulate metal and to provide extruded cansand the like which are substantially free of the tendency to crackadjacent the top.

This and other objects and advantages of the present invention will bemore clearly understood by those skilled in the art upon becomingfamiliar with the following description and the illustrative examples.

It has now been discovered that upon providing magnesium-base alloy oraluminum-base alloy in particulate form wherein at least about /3 of theparticles have a ratio of the longest to shortest dimension of at leastabout 2, compressing a charge of such particulate metal into a compactand impact extruding the compact at an elevated temperature, theresulting impact extruded article is substantially free of any tendencyto crack in the longitudinal direction of extrusion, e.g., from the topof the bottom of an extruded can.

Preferably, in the practice of the invention, at least about 50 percentof the metal particles employed will have a length/width dimension ratioof 2. More preferably, at least about 50 percent of the particles have alongest dimension to shortest dimension ratio of at least about 3. Evenmore preferably, substantially all of the particles have a longest toshortest dimension ratio of at least 5. The best form of particulatemetal to be employed in the practice of the invention appears to beplatelets.

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The particulate material used in the present invention may be made inseveral different ways. Particles may be made by mechanically reducingsolidified cast light metal alloy, for example, an ingot. Mechanicalreduction is accomplished by machining or chipping the cast metal intopieces, flakes or chips. Typically suitable chips are 0.25 inch long and0.013 inch thick.

Particles of magnesium-base alloy or aluminum-base alloy are alsoobtained by jet atomizing or wheel atomizing, as well understood in theart. The resulting pellets usually have particle diameters passing abouta No. 20 to a No. 200 sieve (U.S. Sieve Series). A charge of suchpellets and a quantity of hardened tool steel balls, alumina cylinders,or similar inert, relatively dense objects ordinarily employed for ballmill grinding operations are placed in a suitable vessel such as arotating ball mill or an oscillating paint shaker. When the vessel isrotated, tumbled, or oscillated, the movement of the steel balls orother heavy objects results in a peening or flattening of the pellets.Generally, the pellets are sufliciently flattened in about 15 to 30minutes of motion of the vessel.

In each of several repetitive tests, a 50 gram charge of atomizedpellets formed of the ASTM designated magnesium-base al'loy, ZKlO, wasplaced in an enclosed vessel along with steel balls each /13 indiameter. The vessel was attached to a paint shaker and shaken foreither 5 or 15 minutes. At the end of the period of shaking, thecontents of the vessel were separated from the steel balls with a coarsescreen and graded by pouring successive portions onto an inclined tablewhere rounder pieces rolled away from flatter pieces. Examination of thefractions obtained with the inclined grading table showed that pelletswhich had been peened about 5 minutes consisted of (1) about 34 percentof pellets which were slightly deformed, some being flattened on oneside; (2) about 52 percent of pellets which were considerably moredeformed, about A of the pellets being flat on both sides; and (3) about14 percent of pellets which were generally flat platelets having longestlength/shortest dimension ratios in the range of 5 to 10, typicalparticles having a longest dimension of 0,020" and a shortest dimensionof 0.003.

Pellets which had been peened 15 minutes consisted of (1) about 12percent of pellets which were slightly deformed, some being flattened onone side only; (2) about 54 percent of pellets which were moresignificantly deformed, approximately of the pellets being flat on bothsides; and (3) about 34 percent of the pellets being generally flatplatelets and having dimension ratios in the range of 2 to about 5,typical platelets being 0.003" thick and about 0.010" in greatestlength.

If desired, the pellets employed may also be flattened by passing thepellets between the rolls of a mill arranged for vertically downwardfeeding and the rolls being drawn together very tightly. Such method isfurther described in US. Patent No. 3,179,516.

In a practical way, the average longest dimension to shortest dimensionratio of flattened atomized pellets is indicated by the bulk density ofa quantity of the flattened platelets. The lightest attainable bulkdensity on flattening pellets of these alloys appears to be about 45percent of the theoretical or maximum attainable density. Thetheoretical density of magnesium is 1.74 grams per cubic centimeter. Thetheoretical density of aluminum is 2.70 grams per cubic centimeter. Thetheoretical density for the alloys of magnesium and of aluminum varysomewhat from these values. For the purposes of the invention the bulkdensity of flattened pellets must be no higher than about 60 percent ofsuch theoretical density and more preferably is 55 percent or less ofthe theoretical density. Further, it appears that the longest dimensionto shortest dimension ratios are greater than 10 for at least about halfof the pellets when the bulk density is less than 60 percent of thetheoretical density.

The particulate metal obtained as described hereinabove is compressedinto a compact under the requisite conditions of pressure andtemperature to form a rigid coherent compact having at least 75 percentof the theoretical density of the metal. In general, it is satisfactoryto place a charge of the particles in a container having one end blockedoff, e.g., the container of an extrusion press having the die openingblocked off, and to compress the metal under about 50,000 to 150,000pounds per square inch pressure. Aluminum is ordinarily compressed atambient room temperature, heating being avoided because of lubricationproblems. Magnesium alloy pellets may be compressed cold or hot. In thelatter case, temperatures of the order of 500 to 700 F. are employed.The compact is preheated sufliciently to permit impact extrusion toproceed Without cracking the work piece, generally to an elevatedtemperature above about 300 F. and typically at a temperature in therange of about 500 to 800 F. The preheated compact is then placed overthe die opening and impact extruded at about the preheat temperature ina manner Well understood in the art.

In the event problems are met in forming a coherent compact by coldpressing, it may be found necessary to use freshly made machine chips orfreshly flattened pellets. Also, it may be desirable to store thefreshly peened particles or freshly made chips in a dry, cool place orin a substantially oxygen-free, dry enclosed zone. The pellets arepreferably sufficiently worked to provide a bright new surface.Therefore, the peening of pellets is preferably carried out for a periodof at last 15 minutes. In the case of magnesium, the problem of forminga compact usually may be overcome by simply compacting the magnesium atan elevated temperature above about 500 F.

If desired, the step of compacting prior to impact extruding may beavoided 'by simply placing a charge of the elongated or flattened metalparticles of the invention directly in the die cavity, bringing theparticles to an elevated temperature of at least about 600 F. and impactextruding the charge of particles as a rigid, substantially fullydensified article.

More generally, the extruded article is made by placing a charge of theparticulate metal in the die cavity and compressing the charge with thepunch under the requisite conditions of temperature and pressure to makea coherent rigid mass. Such mass is a compact generally having a lowerdensity than a fully densified compact or extrusion. Impact extrusion isthen carried out as a separate step, using the same common die, ifdesired, or a different die, upon advancing an appropriate size of punchwith sufficient clearance between punch and die for extrusion and underadditional pressure, viz, sufficient pressure to bring about impactextrusion. The common die, e.g., may be used in a turntable arrangementwherein the turntable is rotated to advance the die from a larger to asmaller punch.

The extruded article obtained according to the practice of the inventionis substantially not subject to longitudinal cracking adjacent the topof the article, and wherein cracking does occur at the top of thearticle, it does not propagate far from the top.

The following examples serve to illustrate the invention and do notlimit the scope thereof.

EXAMPLES In each of a series of runs carried out in quadruplicate toillustrate the method of the invention, 50 gram quantities of jetatomized pellets formed of a magnesiumbase alloy having the A.S.T.M.designation ZKlO were, respectively, peened for various periods of timein a paint shaker having a volume of about 900 cubic centimeters andcontaining ninety 45" steel balls. In each case, the bulk densityobtained on peening was determined by measuring the volume of the chargebefore and after peening. The volume measurement was made by observingthe height of the charge when placed in a graduated cylinder and aftertapping the graduated cylinder firmly. Bulk density was computed interms of the density of the pellets as follows:

bulk density:

100 X l gght of unpeened charge) heim density Of as-atom1zed pGllGtSAtomized pellets of magnesium-base alloy are known to have about /3 ofthe theoretical density of the base metal. The percent of theoreticaldensity was computed by multiplying the foregoing percentage by /3.

Each charge of peened pellets was cold compacted under a pressure of 25tons in a 1.175 inch diameter container of an extrusion press having thedie opening blocked off. Each compact, in turn, was placed in the diecavity of an impact extrusion press maintined at a temperature of about700 F. After two minutes of preheating in the die cavity, the compactwas impact extruded into a cylindrical can 1.187 inches in diameter andabout 2.5 inches in length with a side-wall thickness of about 0.062inch and a bottom wall thickness of about 0.10 inch. Each impactextrusion was examined visually for longitudinal cracking adjacent thetop of the can. The peening times, the bulk densities obtained, and theresults of the visual examination of the impact extrusion are summarizedin the following table, bulk densities being averaged for thequadruplicate tests.

Run Peening time, Bulk density, Results on No. minutes percent of impacttheoretical extrusion 2. 5 60 All 4 cracked. 5 64 2 0i 4 cracked. 5 60 3of 4 cracked.

10 60 2 of 4 cracked.

10 54 None cracked.

15 54 1 of 4 cracked.

20 47 None cracked.

The results of runs number 5 to 10 in the table demonstrate theimportance of employing an adequate proportion of the sufficientlyelongated or flattened pellets, in the charge to be compacted and impactextruded, as indicated by the bulk density determination.

In an additional run made by way of comparison, the as-atomized pelletswithout being peened, were employed as a control. In this control study,no coherent compact was formed on cold compressing.

As an additional example of the method of the invention, 50 grams of jetatomized pellets formed of aluminum alloy consisting of about 2 percentby weight of silicon and the balance aluminum were peened for 30 minutesin a paint shaker having a volume of about 900 cubic centimeters andcontaining ninety inch diameter steel balls. The so-trcated charge ofpellets exhibited a bulk density of about 50 percent of the theoreticaldensity for aluminum. The charge of peened pellets was cold pressed intoa compact having a density about percent of theoretical for aluminum.The compact was preheated for 2 minutes by placing it in the dies of anextrusion press maintained at 500 F. Then the compact was impactextruded into a cylindrical can. Visual examination of the cylindricalcan showed no longitudinal cracking adjacent the top thereof.

The method of the invention having been thus fully ticles fromparticulate metal formed of a light metal alloy selected from the groupconsisting of magnesium-base alloys and aluminum-base alloys, bysubjecting a charge of such particulate metal to an impact extrusionoperation at an elevated temperature thereby to form a rigid,substantially fully densified impact extruded article, the improvementwhich comprises: employing as the charge a particulate metal of which atleast /a of the particles have a longest to shortest dimension ratio ofat least about 2.

2. The method as in claim 1 in which the particulate light metal alloyis provided by mechanically reducing solidified cast light metal alloyto chips.

3. The method as in claim 1 in which the particulate light metal alloyis provided by mechanically flattening atomized pellets of light metalalloy.

4. In the method of producing impact extruded ar ticles from particulatemetal formed of a light metal alloy selected from the group consistingof magnesium-base alloys and aluminum-base alloys by compressing acharge of such particulate metal under the requisite conditions oftemperature and pressure to form a rigid coherent compact and subjectingthe resulting compact to an impact extrusion operation at an elevatedtemperature, the improvement which comprises, employing as the charge aparticulate metal of which at least 50 percent of the particles havelongest to shortest dimension ratios of at least about 2.

5. The method as in claim 4 in which the particulate light metal alloyis provided by mechanically reducing solidified cast light metal alloyto chips.

6. The method as in claim 4 in which the particulate light metal alloyis provided by mechanically flattening atomized pellets of light metalalloy.

7. The method as in claim 6 in which the mechanically flattened atomizedpellets have the form of platelets.

8. The method as in claim 4 in which substantially all of theparticulate particles have longest to shortest dimension ratios of atleast about 5.

9. The method as in claim 4 in which at least about /2 of theparticulate particles have longest to shortest dimension ratios of atleast about 3.

10. The method as in claim 4 in which the particulate metal is analuminum-base alloy, and the compressing of said metal into a compact iscarried out at substantially ambient room temperature.

11. The method as in claim 4 in which the particulate metal is amagnesium-base alloy and the compressing of said metal into a compact iscarried out at a temperature in the range of from about 500 to about 700F.

12. The method as in claim 4 in which the steps of first compressing theparticulate metal into a compact and subsequent impact extruding theresulting compact are performed sequentially in a common die.

W lletere nces Cited UNITED STATES PATENTS 2,842,440 7/1958 Nacht -2 14X 3,126,096 3/ 1964 Gerard. 3,160,502 12/1964 Quartullo. 3,178,2803/1965 McGee 75-211 X 3,189,988 6/1965 Crane 29420.5 3,219,490 11/ 1965Foerster 75--226 X 3,276,867 10/ 1966 Brite 75-226 X FOREIGN PATENTS464,727 3/ 1937 Great Britain.

542,268 6/ 1957 Canada.

686,673 5/ 1964 Canada.

706,486 3/ 1954 Great Britain.

768,204 2/1957 Great Britain.

797,577 7/1958 Great Britain.

CARL D. QUARFORTH, Primary Examiner.

A. J. STEINER, Assistant Examiner.

